Poor quality diet and physical inactivity are the most prevalent, preventable causes of death in the United States. In particular, high saturated fat diet (Fat), low fruit and vegetable intake (FV), low physical activity (PA), and high sedentary leisure screen time (Sed) co-occur and heighten the risks of cardiovascular disease and cancers. The first Make Better Choices (MBC1) experiment contrasted four strategies to promote healthful change across these four risk behaviors. Each intervention targeted two behaviors (one diet, one activity) simultaneously and either increased healthy or decreased unhealthy responding. The intervention targeting increased FV and decreased Sed was most efficacious, yielding unexpectedly sustained improvement in three out of the four risk behaviors (FV, Sed, Fat). The proposed MBC2 trial tests the efficacy of MBC intervention to promote sustained, healthful change in diet and activity at 6 and 12 months, as contrasted with an attention control treatment (stress management) . MBC2 also (a) tests competing hypotheses about the optimal way to add PA without undermining maintenance of FV, Sed, and Fat; and (b) examines mediators and biomarkers of healthy lifestyle change. Community dwelling adults (N=250) with suboptimal diet and inactive lifestyle will be randomized to: 1) Sequential MBC (FV+ Sed-, followed by PA+), 2) Simultaneous MBC (FV+Sed-PA+), or 3) Control (stress management). All will use a smart phone with behavioral decision support tools to self- monitor and transmit data and will be coached by telephone. The Mastery hypothesis, based upon theories of self-regulatory strength and habit, predicts superior healthy change for sequential intervention, mediated by greater habit strength for healthy eating and activity. The Synergy hypothesis, based upon goal systems theory, predicts best outcome from simultaneous intervention, mediated by the establishment of a superordinate healthy lifestyle goal. Results of the MBC2 trial will shed light on mechanisms that underlie healthy change in prevalent risk behaviors. If successful, the trial also will result in an innovative, highly disseminable technology-supported minimal counseling intervention to address the modal American unhealthy diet and sedentary lifestyle.